Electrophotography – Image formation – Charging
Reexamination Certificate
2002-10-28
2004-04-27
Tran, Hoan (Department: 2852)
Electrophotography
Image formation
Charging
C399S302000, C399S303000
Reexamination Certificate
active
06728502
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electroconductive member used in an electrophotographic copying machine, printer or the like, and an image forming apparatus using the same.
2. Description of Related Art
In an image forming apparatus using an electrophotographic manner, uniform electric charges are formed (electrification) on the surface of an electrostatic latent image carrier (photoreceptor), and an electrostatic latent image is formed using a laser with modulated image signals. Thereafter, the electrostatic latent image is developed with electrified toner to form a toner image. The toner image is electrostatically transferred, through an intermediate transfer member or directly, onto a recording medium, whereby a desired transferred image can be obtained.
In the transferring manner using the intermediate transfer member, a semiconductive endless belt (semiconductive belt) is adopted as the intermediate transfer member. This semiconductive belt is generally made of an elastic material from the viewpoint of easy control of the driving of the belt, and the other viewpoints. In general, a vulcanized rubber as follows is used: ethylene-propylene-diene rubber (EPDM), urethane rubber, epichlorohydrin rubber.
For example, Japanese Published Unexamined Patent Application No. Hei 2-264277 suggests the use of a belt wherein a polyethylene film is laminated on an EPDM rubber having a volume resistivity of 10
15
to 10
16
&OHgr;cm. Furthermore, Japanese Published Unexamined Patent Application No. Sho 63-83764 suggests a semiconductive belt or the like wherein an elastic body is used as a material having a volume resistivity of about 10
10
to 10
13
&OHgr;cm.
Since the semiconductive belt is made of an elastic body having high elasticity, the belt has a higher tensile strength than elastomers. The transferring semiconductive belt for forming an image is required to have given paper-passing durability as well as a high tensile strength. About this paper-passing durability, a good performance is obtained by setting the thickness of the belt to a given thickness.
In the case in which the following is used in order to obtain such a high tensile strength and given paper-passing durability: for example, the transferring carrying belt described in the Japanese Published Unexamined Patent Application No. Hei 2-264277, having a volume resistivity of 10
15
to 10
16
&OHgr;cm, or the transferring carrying belt described in the Japanese Published Unexamined Patent Application No. Sho 63-83764, having a volume resistivity of about 10
10
to 10
13
&OHgr;cm, there arises a problem that an electric field necessary for the transfer becomes large so that a heavy burden is imposed on a power source for applying voltage to the belt.
Since the transferring carrying belt holds a transferring material (recording medium) through electrostatic absorption power, discharge of electricity may be generated when the transferring material is separated from the belt. In this case, a transfer image on the surface of the transferring material may be disturbed. This discharge is easily generated particularly under environment of a low temperature and low humidity. However, in the case in which the abovementioned semiconductive belt having a high volume resistivity is used as the transferring carrying belt, a high voltage is necessary for holding the transferring material by means of the belt; therefore, discharge of electricity is easily generated. This discharge phenomenon causes some parts of toner on the surface of the transferring material to reverse polarity. Thus, transfer defects are generated so that image defects called voids (white omissions, or drop-outs) are caused in some parts of the transferring material surface. Thus, the transfer image is easily disturbed to result in a problem that a good image quality is not easily obtained.
In the case in which the volume resistivity of the transferring carrying belt is less than 10
2
&OHgr;cm, electric charges flow easily, thereby resulting in a problem that the transferring material cannot be held through electrostatic absorption power.
Japanese Published Unexamined Patent Application No. Hei 8-185068 suggests, as a transferring carrying belt, a belt wherein the surface of an elastic body having a volume resistivity of 10
9
&OHgr;cm or less, such as chloroprene, is coated with a nylon resin or an urethane resin. In the case in which the surface layer of the chloroprene is coated with the nylon resin, the surface coat layer cannot follow deformation of the transferring carrying belt at a curvature portion, where the belt passes through a roll-shaped support, since the coat layer is hard. Thus, a crack may be generated in the surface layer. In the case of the urethane resin coat, the coat layer has flexibility not to cause a problem of the generation of a crack on the coat layer surface as described above. However, this belt has a problem that toner adheres easily to the surface so that the surface easily gets dirty.
In the case in which an elastic body such as chloroprene in which carbon black or the like is dispersed is used for the transferring carrying belt, a desired stable resistance value can hardly be obtained even if ordinary conductive carbon black is added to ordinary rubber material. This is because a semiconductive resistance range having a resistivity of about 10
9
&OHgr;cm is a range wherein resistivity is not easily controlled. As a result, it is difficult that variation in the resistance of the belt using the abovementioned elastic body is stably set in such a manner that the common logarithm of the volume resistance thereof is within one figure (digit). In the case in which in-plane variation in the resistance is one figure or more, transferring voltage cannot be uniformly applied. Therefore, a problem arises that image quality after transfer is not stable.
In order that the semiconductive belt holds the transferring material and a toner image is transferred onto the transferring material, a transferring voltage of 1 to 5 kV is applied. This applied voltage causes a change in the resistance of the belt material, resulting in a problem that a difference in the resistance value of the belt is generated between the region where the transferring material is present and the region where it is not present.
To overcome a change over time in the resistance value of the abovementioned belt material and unevenness of the belt resistance value, Japanese Published Unexamined Patent Application No. Hei 8-292648 suggests a transferring carrying belt formed of three layers wherein the volume resistivity of the first layer (surface layer) is set within the range of 1×10
10
to 1×10
16
&OHgr;cm; a rubber layer using the conductivity of a polymer itself, the volume resistivity range thereof being from 1×10
7
to 1×10
10
&OHgr;cm, is used as the second layer (intermediate layer); and the volume resistivity of the third layer (base layer) is set within the range of 1×10
10
to 1×10
16
&OHgr;cm. This publication states that the unevenness of the resistance value can be improved by the rubber layer using the conductivity of the polymer itself, which is the second layer (intermediate layer). However, in the case of the laminated belt, the resistance value thereof is controlled by the layer having a high resistance value. Thus, the unevenness of the resistance value is not sufficiently overcome.
Japanese Published Unexamined Patent Application No. Hei 9-179414 suggests a rubber material made of chloroprene rubber and EPDM (ethylene propylene diene monomer) as a countermeasure against the change over time in the resistance of the belt material. However, this suggestion is insufficient for overcoming this change over time and the unevenness of the belt resistance.
Furthermore, Japanese Published Unexamined Patent Application No. 7-271204 suggests the use of an ion-conductive type rubber material made of a rubber material having an intense polarity, such as hydrin rubber, as
Fuji 'Xerox Co., Ltd.
Oliff & Berridg,e PLC
Tran Hoan
LandOfFree
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